1. Field of the Invention
This invention relates to a double-slide valve for use in controlling the flow of particulate material, and more particularly, for use in controlling the flow of finely divided solid catalyst in fluid catalytic cracking units.
2. Description of the Prior Art
In the operation of a fluid catalytic cracking unit, large quantities of finely divided solid catalyst are circulated through the unit in a fluidized state. This finely divided solid catalyst must continuously pass between regenerator and reactor, and through supplementary equipment at very high flow rates, up to one hundred tons per minute, and in closely controlled quantities in order to establish and maintain the desired operating conditions, e.g., catalyst to oil ratio in the reactor, reactor temperature, catalyst residence time in the reactor, and the residence time and temperature in the regenerator. The flow of this finely divided fluidized solid catalyst is controlled by valves placed in the fluid catalyst transfer lines, i.e., the regenerator effluent standpipe for control of flow of regenerated catalyst to the reactor and the reactor effluent standpipe for control of flow of spent catalyst to the regenerator, and in supplementary equipment as for example a regenerator standpipe leading to recycle steam generators. These valves are normally installed in pairs wherever control is required. Usually, the upper valve of each pair is maintained as a safety reserve in case of failure of the lower operating valve.
Such conventional valves, presently in use in fluid catalytic cracking units, are slide valves which include a valve body, an inlet liner or port, a slide plate or disc, and actuating means to position the slide plate with respect to the liner or port to control the flow of the solid catalyst. The slide plate or disc is normally positioned in the slide valve on the downstream side of the flow port. Support or alignment guides are provided to hold the slide plate in this position and to allow the slide plate to be moved in a plane perpendicular to the flow of catalyst in order to control the rate of catalyst flow.
During the passage of large quantities of catalyst through these slide valves, erosion occurs because the restrictive opening increases the velocity of the catalyst. This increased velocity causes the leading edges of the slide plate and the edge of the flow port to wear extensively. This erosion becomes so severe that it requires a shutdown of the unit so that a replacement of the slide plate and port can be made.
Because of the erosion caused by the finely divided solid catalyst stream, valve design for flow control of finely divided solid catalyst presents a serious problem and a problem which is not encountered in normal flow control design or operation with liquids or gases. In normal catalytic cracking operations, the usual life of a slide valve can range from three months to three years. This range varies depending upon whether the slide valve is located on a regenerator standpipe where the temperatures can approach 1,400.degree. F. or whether the valve is located on a reactor standpipe where the temperature is in the range of 800.degree. F.-1,050.degree. F. Also, lower velocities of the solid catalyst and the use of different types of liners, such as a hex-steel liner which contains a refractory material, may prolong the life of the valve. However, no present design or configuration has been able to withstand the erosion effect of the catalyst stream for a period substantially longer than three years.
The general object of this invention is to provide a double-slide valve for use in controlling the flow of particulate material. A more specific object of this invention is to provide a double-slide valve for use in controlling the flow of finely divided solid catalyst in fluid catalytic cracking units.
A further object of this invention is to provide a double-slide valve which will stand up to wear and erosion longer than existing slide valves thereby requiring fewer shutdowns of the units in which they are used.
Another object of this invention is to provide a cost savings on a catalytic cracking unit by replacing two valves with a single valve which requires fewer shutdowns of the unit and a less likely chance that equipment failure will occur.
Still another object of this invention is to reduce the number of valves needed in a standpipe thereby reducing the amount of pressure drop across the valves.
Other objects and advantages will become apparent to one skilled in the art based upon the ensuing description.